Spatial coherence of anisotropic and astigmatic sources in interference electron microscopy and holography

The lateral coherence properties of two illumination modes in electron interferometry and holography are investigated within the framework of the mutual coherence function. It is shown that the results obtained can be considered particular realizations of the general, anisotropic, Gaussian Schell model, which plays an important role in the classical coherence theory in optics. Another property of this model is described: the ratio between the coherence and illumination areas is constant along every section of the beam. This invariant parameter is shown to be related to another geometric optical invariant of the beam, i.e., its etendue. Numerical calculations showing the relation between illumination and coherence areas in the image, Fresnel, and far-field (Fraunhofer) regions are presented and discussed with regard to the experimental implications.